Place a saucepan over medium heat and add the butter. Cook, swirling gently, until it turns a golden brown and emits a nutty aroma, about 3-4 minutes. Remove from heat and let cool slightly.
Pour the browned butter into a large mixing bowl. Add the brown sugar and whisk together until smooth and glossy, about 2 minutes. The mixture should be slightly thick and fragrant.
Crack in the eggs one at a time, whisking well after each addition. Incorporate the vanilla extract and continue whisking until the mixture is thickened and well combined.
Sift the flour into a separate bowl to ensure even distribution. Gradually fold the flour into the wet mixture using a spatula, mixing just until combined. Be gentle to keep the blondies tender.
If using, fold in the chocolate chips or nuts, distributing them evenly throughout the batter.
Line your 9-inch square baking pan with parchment paper, leaving a slight overhang for easy removal. Pour the batter into the prepared pan and spread it out evenly with a spatula, tapping the pan lightly to remove air bubbles.
Bake in a preheated oven at 175°C (350°F) for 22-25 minutes. The blondies are done when the edges are golden and a toothpick inserted into the center comes out with moist crumbs.
Remove from the oven and let cool in the pan for at least 15 minutes. Then, lift the blondies out using the parchment overhang and transfer to a wire rack to cool completely.
Cut into squares once cooled, revealing a chewy, golden interior with a slightly crisp top. Serve and enjoy the rich, toasted aroma with each bite.
